This invention relates to flow mixers for mixing concentric fluid streams. More particularly, this invention relates to improved apparatus for efficiently and rapidly mixing the primary and bypass streams of a gas turbine engine of the turbofan variety.
As is known in the art, a turbofan engine includes a gas generator which supplies a primary fluid stream comprising high energy combustion products and at least one fan stage which compresses ambient air to supply a secondary stream of "by-pass air".
Many of these turbofan engines are of the common exhaust variety wherein the primary fluid stream and the secondary fluid stream merge within a common exhaust duct and are exhausted into the atmosphere to produce forwardly directed thrust. In such a turbofan engine, the gas generator is included within a core engine to produce high temperature, high pressure, combustion products that are expanded through one or more turbine stages to develop shaft horsepower for rotating one or more high pressure axial compressor stages and one or more fan stages. The fan stages, which receive ambient air, act as a low pressure compressor to deliver air to the inlet of the high pressure axial compressor stages, which, in turn, compress the air for delivery to the gas generator. A portion of the airstream delivered by the fan stages is bypassed around the engine core such that the bypass or fan air coaxially surrounds the combustion products or primary flowstream as the two flowstreams pass rearwardly into a common exhaust duct. The two flowstreams mix partially with one another within the exhaust duct and flow therefrom to produce forwardly directed thrust.
If thorough mixing of the two fluid streams is to be achieved within a relatively short distance, such common exhaust turbofan engines require a mixer assembly that either forms the terminating portion of a common boundary wall that separates the two fluid streams from one another, or is mounted directly downstream of the terminating edge of such a common boundary wall.
The mixing of the fan air with the turbine exhaust gasses can increase the engine thrust over that which would be produced by exhausting unmixed fluid streams having a substantially identical exhaust pressure, total mass flow, and total energy. This increase in thrust arises since the mixed gasses have a higher mass-velocity product than that which would be produced by separately exhausting the two fluid streams. Further, and of prime significance with respect to this invention, mixing the fan air with the turbine exhaust gases can decrease engine noise. More specifically, one component of gas turbine noise commonly called "jet noise" results from pressure disturbances created at the flow boundaries or interface between a high velocity jet stream and the surrounding atmosphere. In this respect it has been determined that the jet noise produced by a high velocity, singlestream discharge is proportional to the relative velocity between the jet stream and the ambient atmosphere exponentially raised to a relatively high power (typically 8), In a turbofan engine wherein the primary and fan air streams are not caused to thoroughly mix with on another within the common exhaust duct, the velocity of the primary fluid stream is typically 1.4 to 1.7 times greater than the velocity of the fan air stream. Consequently the major contribution to the jet noise is caused by the relatively high velocity primary fluid stream. On the other hand, mixing the primary fluid stream with the fan air within the common exhaust duct substantially reduces the velocity of the thrust-producing fluid stream that is discharged into the atmosphere, substantially reducing the jet noise.
A variety of mixer arrangements for forcing the two fluid streams to mix with one another prior to discharge into the atmosphere are known to the art. In general, these prior art mixers are arranged to reconfigure the cross-sectional flow pattern of the two fluid streams such that, when the fluid streams pass rearwardly over the aft terminus of the mixer, the area of the interface between the two fluid streams is increased over the interface area which would be present if the two streams were discharged in coaxial relationship with one another. Increasing the interface region between contiguous portions of the two flow streams effectively brings a greater portion of each fluid stream into "contact" with the other fluid stream and increases mixing since axial shear forces are introduced throughout such boundary or interface regions. Additionally, such prior art mixing apparatus may impart a radial velocity component to both fluid streams that further enhances mixing when the two fluid streams flow rearwardly from the aft terminus of the mixer. For example, in an arrangement disclosed in U.S. Pat. No. 3,289,413 issued to W. B. Gist, Jr., a mixer is arranged to effect an inwardly directed radial velocity component in portions of the fan airstream and an outwardly directed radial velocity component in interspersed portions of the primary stream as two flowstreams pass axially along the mixing apparatus. Aft of the mixer terminus, these oppositely directed radial components effectively cause radial shear forces at the interface between the two flowstreams and hence cause the two fluids to mix with one another.
The most widely known prior art mixer is commonly called a lobed mixer or daisy mixer. A daisy mixer includes a tubular mixer section having a number of axially extending, circumferentially spaced-apart lobes or corrugations of increasing radial dimension relative to the mixer length. This mixer section is coaxially mounted within a generally cylindrical outer duct and, in gas turbine engines which include a rearwardly extending tail plug, coaxially surrounds the tail plug. The primary fluid stream, exhausted by the turbine stages, flows through an annular passage formed between the exterior surface of the tail plug and the interior surface of the mixer section and the fan air flows through an annular passage formed between the inner surface of the outer duct and the outer surface of the mixer section. The outer duct extends rearwardly beyond the aft terminus or mixing plane of the mixer section to form a common exhaust duct or mixing chamber. As the fluid streams flow along the mixer section, the cross-sectional pattern of the duct containing the primary fluid stream takes on a generally star-shaped or daisy petal geometry. Thus, as the two fluid streams come together at the mixer section exit plane, the primary fluid stream effectively includes a plurality of radially extending, rearwardly directed fluid streams interspersed betweeen regions of flowing fan air. The mixing of the two fluid streams is enhanced by the increased interface area between the two fluid streams and occurs as the fluid streams travel through the common exhaust duct or mixing chamber.
Several disadvantages and drawbacks are experienced with respect to prior art mixers such as the daisy mixer. First, since the mixer section must redirect the two flowstreams to form flow patterns having a relatively large interface area between contiguous portions of the flowstreams, and since no mixing occurs prior to the point at which the flowstreams pass over the aft terminus or exit plane of the mixer section, the prior art mixers must be of a considerable axial length if thorough mixing is to be achieved. Such substantial axial length not only imposes design constraints relative to new engine configurations, but often prevents the design of improved noise suppressing mixers for utilization with an existing engine installation. With respect to gas turbine powered aircraft, such retrofit designs are of increasing importance since governmentally imposed noise limitations must often be met not only by newly designed engines, but also by the engines of aircraft presently in service. In this regard, if suitable noise abatement modifications cannot be effected on existing aircraft engines, these engines can become totally unusable, or at the very least, limited in service application. Further, the relatively long prior art mixers are susceptible to failure induced by metal fatigue arising at least in part because of the high temperature environment and vibration incumbent in the operation of a gas turbine engine. Specifically, since the lobes of a daisy mixer have relatively flat sidewalls with little or no pressure differential being produced across the sidewalls during operation of the engine, the sidewalls are especially susceptible to flutter and fatigue. Additionally, due to the substantial axial length, such prior art mixers are generally quite heavy and thus can cause a weight penalty that detracts from the operating efficiency and overall performance of the aircraft. Even further, both the intial fabrication cost and the repair costs for such prior art mixers are relatively high due to the complex geometry and welded seams necessary therein.
Other performance penalities often result from the use of prior art mixers. For example, since the necessary redirection of the fluid streams to effect the interspersion of the primary fluid stream with the fan air takes place over the relatively long mixer section, substantial pressure losses are often caused in both fluid streams. These pressure losses can prevent attainment of the previously described gain in thrust that is theoretically possible by mixing the fluid streams. In fact, such pressure losses may result in less thrust than is potentially available in a comparable turbofan engine of the unmixed flow variety. Further, each gas turbine engine is designed to operate with a predetermined thermodynamic cycle. Thus to prevent performance degradation, a mixer section must match the engine cycle of the particular engine in which the mixer is employed. With respect to prior art mixers such as the daisy mixer, the engine cycle matching characteristics are primarily determined by discharge coefficients that are associated with the cross-sectional geometry of the mixer exit plane. Since the crosssectional geometry of the exit plane is rather complex, these discharge coefficients of the prior art mixers cannot generally be determined by theoretical considerations and a great deal of experimental effort is generally necessary to determine the optimum mixer configuration. Thus, although prior art mixers such as the daisy mixer have been subject to a great deal of experimental and developmental effort to overcome the above described disadvantages and drawbacks, widespread practical application of these mixers to gas tubine powered aircraft has not been achieved.
Accordingly, it is an object of this invention to provide mixing apparatus of relatively short axial length for thoroughly and efficiently mixing two concentric fluid streams.
It is another object of this invention to provide a gas turbine mixer for mixing fan air with primary combustion gasses to greatly decrease the jet noise produced by the gas turbine engine.
It is yet another object of this invention to provide a relatively lightweight flow mixer of relatively short axial length that is usable within a variety of existing and newly-designed gas turbine engines.
It is still another object of this invention to provide a mixer for a gas turbine engine of the turbofan variety wherein the mixer has minimal effect on the engine cycle match.